Radio frequency (“RF”) switches are used widely in microwave and millimeter wave transmission systems for antenna switching applications including beam forming phased array antennas. In general, such switching applications presently use semiconductor solid state electronic switches, such as Gallium Arsenide (“GaAs”) MESFETs or PIN diodes, as contrasted with mechanical switches. Such semiconductor solid state electronic switches also are used extensively in cellular telephones for switching between transmitting and receiving.
When RF signal frequency exceeds about 1 GHz, solid state switches suffer from large insertion loss in the “On” state (i.e., when an electrical signal passes through the switch) and poor electrical isolation in the “Off” state (i.e., when the switch blocks transmission of an electrical signal). MEMS switches offer distinct advantages over solid-state devices in both of these characteristics, particularly for RF frequencies near or exceeding 1 GHz.
U.S. Pat. Nos. 5,994,750, 6,069,540 and 6,535,091 all disclose MEMS switches in which a pair of coaxial torsion bars, a pin or a pair of flexible hinges support respectively substantially planar and rigid beams or a vane for rotation about an axis established by the torsion bars, pin or flexible hinges. In all three patents, the pair of coaxial torsion bars, the pin or the pair of flexible hinges respectively support the substantially planar and rigid beams or vane a small distance above a substrate. U.S. Pat. No. 5,994,750 (“the '750 patent”) discloses that ends of the torsion bars projecting outward from the beam and anchored respectively to a pair of support members alone support the beam the small distance above the glass substrate. Both U.S. Pat. No. 6,069,540 (“the '540 patent”) and U.S. Pat. No. 6,535,091 (“the '091 patent”) interpose respectively the pin or an upper and lower fulcrum located at the flexible hinges between the beam or vane and the substrate to maintain a spacing therebetween.
In the instance of the '750 patent, the beam extends to only one side of the torsion bars so its rotation thereabout in closing an electrical switch provided thereby is equivalent to the movement of a door swinging on its hinges. Alternatively, both in the '540 and '091 patents the respective beam or vane extends in both directions outward from the pin or pair of flexible hinges. Thus in the structures respectively disclosed in these two patents, in closing an electrical switch the beam's or vane's rotation about the axis established by the pin or pair of flexible hinges resembles the movement of a seesaw. In all three patents, electrostatic attraction induces rotation which effects switch closure.
Omitting numerous fabrication details which appear in the text and drawings of the '750 patent, it discloses in a first example that material forming its beam initially begins as part of a monolithic p-type silicon substrate which carries an n-type diffusion layer into which boron ions are injected to form a p+ surface layer. That is, the n-type diffusion layer separates the p+ surface layer from the p-type silicon substrate. During the beam's fabrication, etching removes the p-type silicon substrate leaving only material of the n-type diffusion layer and p+ surface layer to form the beam. Similarly, torsion bar fabrication removes material of the n-type diffusion layer leaving only material of p+ surface layer to form the torsion bars. Subsequent processing forms aluminum support members spanning between the p+ surface layer material forming the torsion bar ends and the adjacent glass substrate.
The '540 patent discloses that to reduce switch insertion loss as well as improve sensitivity, its beam is preferably formed from entirely of metal as is the pin about which the beam rotates. In particular, the '540 patent discloses that the beam may be formed from nickel (“Ni”) electroplated at low temperatures compared to most semiconductor processing. The '540 patent discloses that not only does its all metal beam reduce insertion losses relative to known SiO2 or composite silicon metal beams, such a configuration also improves the third order intercept point for providing increased dynamic range. Electrical potentials applied respectively between a pair of gold electrodes deposited on one side of the glass substrate nearest to the metallic beam and a pair of field plates disposed on the opposite side of the glass substrate furthest from the beam generate the electrostatic force which effects rotation of the beam about the metallic pin.
The vane included in the MEMS switch disclosed in the '091 patent is formed of relatively inflexible material, such as plated metal, evaporated metal, or dielectric material on top of a metal seed layer. Thin flexible metal hinges connect opposite sides of the vane to a gold frame which projects outward from the low-loss microwave insulating or semi-insulating substrate. The substrate may be fabricated from quartz, alumina, sapphire, Low Temperature Ceramic Circuit on Metal (“LTCC-M”), GaAs or high-resistivity silicon. Configured in this way, the vane and the hinges are disposed above the substrate, and the flexible hinges electrically couple the vane to the frame. The hinges, which can be flat or corrugated, allow the vane to rotate about a pivot axis that is parallel to the substrate and above the lower fulcrum. Pull-back and pull-down electrodes, which can be encapsulated with an insulator such as silicon nitride (Si3N4), are formed on the substrate adjacent to the vane. Electrical potentials applied either to the pull-down or the pull-back electrodes respectively close or open the MEMS switch.
A series of U.S. Pat. Nos. 5,629,790, 5,648,618, 5,895,866, 5,969,465, 6,044,705, 6,272,907, 6,392,220 and 6,426,013 all disclose MEMS structured which are reminiscent to a greater or lesser extent to those described above for the '750, '540 and '091 patents. These patents all disclose an integrated, micromachined torsional scanner, which in a particular configuration, may include a frame-shaped reference member. A particular configuration of the torsional scanner includes a pair of diametrically opposed, axially aligned torsion bars that are coupled to and project from the reference member. In a particular configuration, a plate-shaped dynamic member, analogous to the beams and vane disclosed respectively in the '750, '540 and '091 patents, is encircled by the frame and is coupled thereto by the torsion bars. Configured in this way, the torsion bars support the dynamic member for rotation about an axis that is collinear with the torsion bars. The reference member, the torsion bars and the dynamic member are all monolithically fabricated from a semiconductor layer of a silicon substrate. A desirable method for fabricating the torsional scanner uses a Simox wafer, or similar wafers, e.g. a silicon-on-insulator (“SOI”) substrate, where the thickness of the plate is determined by an epitaxial layer of the wafer. As compared to metals or polysilicon, single crystal silicon is preferred both for the plate and for the torsion bars because of its superior strength and fatigue characteristics. These patents also disclose using electrostatic force to effect rotary motion of the dynamic member.